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Gravimetry

Gravimetry

At the Conrad Observatory two different techniques are applied: (1) temporal gravity variations are continuously monitored by relative gravimeters; and (2) absolute gravity values are determined multiple times a year for long-term monitoring and also used to calibrate relative gravimeters.

Gravimetry is a geophysical method to measure the gravity field of the Earth. By determining the strength of gravitational acceleration at different locations and/or as a function of time, conclusions can be drawn about distribution and variation of density within the Earth. The determination of the gravity field is of big importance for a series of subject fields like geophysics, physical geodesy, fundamental physics and metrology. This information can be used to constrain global models of the Earth as well as the density structure on smaller scales. In addition, it contributes to our understanding of mass transport phenomena within our planet, in the oceans and atmosphere. Gravimetric measurements aim to provide precise and coherent gravity standards as well as valuable data to calculate gravity anomalies and models for geoid models.

In 1960, detailed profiles crossing the alpine orogen were undertaken to investigate the underground structure of the Eastern Alps. The first aerial gravity map of Austria was published in 1965 by E. Senftl, representing a major landmark in gravimetric research in Austria. Gravimeters are very complex and expensive instruments. For this reason Austria's first absolute gravimeter (JILAg-6) was funded by a number of Austrian research institutions (IMGW, ZAMG, IG-MUL, GBA, ITG-TUG, IAGP-TUG, IWF-ÖAW, and IGG-TUW) and was maintained by the BEV since 1986. Periodical absolute gravity measurements are performed at stations of the Austrian Gravity Base Net (Österreichisches Schweregrundnetz ÖSGN), but also at stations of the European Combined Geodetic Network (ECGN). In a co-operation between ZAMG, IMGW and BEV a combination of the absolute gravimeter (JILAg-6) and a superconducting gravimeter (GWR C025) is used for gravimetric research since 1995. In 1997 ZAMG and IMGW joined the Global Geodynamics Project (GGP) of globally distributed standardized superconducting gravimeters (GGP stations). The superconducting gravimeter was operated in Vienna between 1995 and 2007.

(1) The superconducting gravimeter GWR SG CT-C025 (SG, Fig. 1) was moved to the Conrad Observatory in autum 2007 and is operated there in close cooperation with the University of Vienna (IMGW Gravimetry). As formerly in Vienna, it permanently monitors the temporal gravity variation field as well as seismic modes. Superconducting gravimetry is currently the most precise and most effective tool for investigating temporal gravity variations. Such variations can be caused by various geophysical/geodynamical processes such as earth tides, Earth's rotation variations, translational modes of the inner core, seismic normal modes, atmospheric and ocean loading, hydrological mass flow. The identification and modeling of environmental effects on gravity is essential for extracting meaningful geodynamical signals from gravity time series. However, superconducting gravimeters are relative instruments and have to be corrected for instrumental drift. Absolute gravimeter are used as reference at the same station as well as for scale factor determination. Such calibration experiments happen several times a year.

(2) Absolute measurements of Earth's gravity field are conducted by the Federal Office of Metrology and Surveying (BEV Gravimetry). Absolute gravimetric measurements are taken by transportable absolute gravimeters which measure the free fall of a testing mass in a vacuum chamber with high accuracy. These measurements aim to realize and maintain a precise and coherent standard of gravity for Austria as well as providing basic data for the calculation of gravity anomalies and geoid models. Until 2009 the absolute gravimeter JILAg-6 was in use, which was replaced by a new absolute gravimeter FG5 (AG, Fig. 2) in 2010. The FG5 gravimeter was acquired in cooperation between BEV and ZAMG.